This invention relates to resonant tube level sensors. More particularly, this invention relates to the use of acoustic waves for point level sensing, continuous level sensing, and pressure sensing.
It is often necessary to determine (sense) the level of a material. In general, there are two main types of level sensing: point level sensing and continuous level sensing. Point level sensing determines whether a material is above or below a particular level, while continuous level sensing determines the material level relative to a reference point. It is possible to approximate a continuous level sensor using many point level sensors. Level sensors can be used to determine other parameters. For example, a manometer is a pressure sensor that uses the level of a liquid to determine pressure.
Numerous types of level sensors are known. For example, some level sensors use floats, tuning forks, electrical conductivity, ultrasonic time-of-flight, microwaves, optical reflections, tapes, magnetostriction, capacitance, electromagnetic time domain reflectometry, thermal conductivity, and pressure. In U.S. patent application Ser. No. 09/854,500, entitled xe2x80x9cResonant Tube Level Sensor,xe2x80x9d filed on May 14, 2001, I described a point level sensor based on acoustic resonance in a tube. The patent application is hereby incorporated by reference. Also, reference can be made to U.S. Pat. No. 5,128,656, entitled xe2x80x9cLevel Detecting Method and its Apparatusxe2x80x9d issued on Jul. 7, 1992 to Watanabe for another systems that uses acoustic waves.
While all of the foregoing level sensors are useful, none is optimal in every application. For example, floats are often rather large and are subject to leaks and other failures, tuning forks thermal conductivity, and electrical conductivity sensors are sensitive to material build-up or are limited to certain types of materials, tapes are subject to breakage and require an operator, and magnetostriction, capacitance, ultrasonic, time domain reflectometry, and pressure are typically relatively expensive. While acoustic methods are very promising in that low cost, accurate systems that are relatively insensitive to material build-up and foams are achievable, some problems are evident. Problems with U.S. Pat. No. 5,128,656 and U.S. patent application Ser. No. 09/854,500 will now be discussed.
U.S. Pat. No. 5,128,656 and U.S. patent application Ser. No. 09/854,500 are both based on the physics of resonant tubes. A tube having an effective length L that is filled with a medium (usually a gas such as air) having a speed of sound of c can produce two different sets of resonant frequencies. If the tube is closed at both ends (or open at both ends) the possible resonant frequencies are:
fnc=nc/2L, where n=1, 2, 3,
If the tube is open at one end and closed at the other end the possible resonant frequencies are:
fno=(2nxe2x88x921)c/4L, wherein n=1, 2, 3,
It should be noted that the length L is the effective length. As is well known, the resonant frequency of a tube is subject to an xe2x80x9cend effectxe2x80x9d in that an open tube is acoustically longer than the actual length, with the additional length depending on the radius of the tube""s opening.
Since acoustic resonance is a fundamental physical property, its use in level sensing is beneficial. However, U.S. Pat. No. 5,128,656 appears to have drawbacks in that its method of sensing resonance is not particularly easy to implement, it may have operational reliability problems, and it appears to be difficult to use with caustic vapors. Furthermore, the sensor described in U.S. patent application Ser. No. 09/854,500 did not perform well enough over time and temperature for most practical commercial applications.
One problem with the sensor described in U.S. patent application Ser. No. 09/854,500 was that it worked well only with certain tube lengths. Furthermore, those tube lengths depended on the tube type. For example, copper tubes, plastic tubes and tubes having different wall thicknesses and inner diameters worked best with different tube lengths. Furthermore, once acceptable operation was achieved with a given tube, temperature changes (say by 10xc2x0 F.) made operation erratic. Operation also tended to change over time (say 24 hours). Another problem was that system operation did not always follow the simple physical theory described above. For example, when acceptable operation was achieved with a xe2x80x9cclosed endxe2x80x9d tube, changing frequency to obtain closed resonance at n+1 did not always work. However, the sensor described in U.S. patent application Ser. No. 09/854,500 is highly advantageous in that it has no moving parts (except for the transducer movement), is easy to fabricate, is low cost, rugged, and is difficult to clog.
Therefore, an improved acoustic resonance level sensor would be beneficial. Particularly beneficial would be an improved level sensor that operates on the principles described in U.S. patent application Ser. No. 09/854,500. Furthermore, a new level sensor that extends the principles of acoustic resonance as described in U.S. patent application Ser. No. 09/854,500 to continuous level sensing would be beneficial. Additionally, a new level sensor that extends the principles of acoustic resonance to sensing other parameters, including pressure, would be highly beneficial.
The principles of the present invention provide for point level sensors and for continuous level sensors that can sense the level of a material. Advantageously, the principles of the present invention enable sensing of many materials, including very light solids, such as feather, cotton, and powders, and of almost all liquids, including highly viscous liquids that tend to cling. Additionally, the principles of the present invention enable both temperature and pressure sensing.
A point level sensor according to the principles of the present invention includes a tube having a sense position (such as the end of the tube) and an acoustic assembly that produces sound in the tube. The acoustic assembly is mounted to reduce or eliminate vibrations in the tube body. A beneficial way of reducing or eliminating such vibrations is to use a vibration dampening material, such as a rubber compound, between the tube and a source of acoustic waves. A driver circuit can then drive the acoustic assembly in an attempt to produce a standing wave in the tube at the sensing position. After a time sufficient to produce a standing wave the driver circuit stops driving the acoustic transducer. An electronic network then monitors the decay of the acoustic waves in the tube to determine if a standing wave was produced. Based on that determination, a signal is produced that indicates whether a material has reached the sensing position. Multiple acoustic frequencies can be used to attempt to produce resonance. Beneficially, the acoustic frequency (or frequencies) that are used in the attempt(s) to produce resonance depends on temperature.
According to one embodiment of the present invention, the driver circuit drives the acoustic assembly with a frequency that would produce a standing wave if an end of the tube is open (the end of the tube then being the sensing position). If a standing wave is produced, as determined by the acoustic decay, a level signal is produced that indicates that a material has not reached the end of the tube. Beneficially, the system compensates for temperature effect on the speed of sound (which impacts on acoustic resonance).
According to another embodiment of the present invention, the driver circuit drives the acoustic assembly with a frequency that would produce standing waves if an end of the tube is closed (the end of the tube again being the sensing position). If a standing wave is produced, as determined by the acoustic decay, a level signal is produced that indicates that a material has reached the sensor end. Beneficially, the system compensates for temperature effect on the speed of sound (which impacts on acoustic resonance).
According to another embodiment of the present invention, the driver circuit drives the acoustic assembly with a frequency that would produce a standing wave if the material has reached a sense position inside the tube end. If a standing wave is produced, as determined by the acoustic decay, a level signal is produced that indicates that the material has reached the sense position. Beneficially, the system compensates for the effect of temperature on the speed of sound (which impacts on acoustic resonance).
In any of the foregoing embodiments, beneficially, the driver circuit drives the acoustic assembly at a plurality of possible resonant frequencies in an attempt to produce a standing wave. This enables temperature compensation and provides an easy method of compensating for changing vapor concentrations in the tube.
The driver can also drive the acoustic assembly at a frequency that cannot produce a standing wave (resonance). In that case, the presence of a standing wave is determined by comparing the acoustic decay in the tube at the possible resonant frequency against the acoustic decay in the tube at the frequency that cannot produce resonance.
Also beneficially, the driver circuit drives the acoustic assembly in an attempt to produce resonant when the sensor end is open and then when the end is closed. This enables a fail-safe approach in that the tube must be either open or closed (neglecting a small xe2x80x9ctransition rangexe2x80x9d where the tube is neither open nor closed). Beneficially, the system compensates for the effect of temperature on the speed of sound (which impacts on acoustic resonance).
The foregoing point level sensors can be used as a point level pressure sensor. To do so, the level of a material is arranged to depend on pressure (a manometer), and a sensing pressure is predetermined. Then, a sensing position is determined, with the sensing position being dependent on the level of the material in the tube when the sensing pressure is reached. Then, the point level pressure sensing system attempts to find the material level at the sensing position using the decay of a standing wave produced in the tube. When the material level reaches the sense position a determination is made that the pressure has reached the sensing pressure and a pressure signal is produced. Beneficially, the system compensates for the effect of temperature on the speed of sound (which impacts on acoustic resonance).
The principles of the present invention further provide for a continuous level sensor that senses the position of a material in a tube. A continuous level sensor according to the present invention uses multiple acoustic frequencies in attempts to induce a standing wave in a tube. The continuous level sensor includes an acoustic assembly for producing sound in the tube. Beneficially, the acoustic assembly is mounted to reduce or eliminate vibrations in the tube body. A beneficial way of reducing or eliminating such vibrations is to use a vibration dampening material, such as a rubber compound, between the tube and a source of acoustic waves. A driver circuit can then drive the acoustic assembly using multiple frequencies in an attempt to produce a standing wave. A sensing network then monitors the decays of the acoustic waves in the tube to determine if a standing wave was produced. Based on those acoustic decays, the frequency that produced a standing wave is found, then the level of a material in the tube is determined from that frequency. Beneficially, the continuous level sensor compensates for temperature effect on the speed of sound (which impacts on acoustic resonance).
The foregoing continuous level sensor can be used to measure temperature. A temperature sensor according to the principles of the present invention includes an acoustic assembly at an end of a tube (which can be either open or closed). Then, acoustic resonance in the tube is established. The temperature is then determined from the tube length and the resonate frequency.
The foregoing continuous level sensor also can be used as a pressure sensor. To do so, the level of a material is arranged to depend on pressure (a manometer). Then, the sensing system finds the material level, and from that material level, determines pressure from the frequency that produces a standing wave in the tube. Beneficially, the system compensates for the effect of temperature on the speed of sound (which impacts on acoustic resonance).
The principles of the present invention further provide for a method of determining whether a material being sensed has reached a predetermined level. In such a method, a sensor end of a tube is located at the predetermined level. An acoustic frequency attempts to create standing waves within the tube, but not in the body of the tube. The acoustic frequency is stopped, and the acoustic decay in the tube is monitored. A determination is then made as to whether standing waves occurred, and, based on that determination, a signal is produced that identifies whether the material being sensed has reached the predetermined level.
The principles of the present invention further provide for a method of determining the level of a material in a tube. In such a method, an end of the tube is located such that material has a level in the tube. Acoustic frequencies attempt to create standing waves within the tube. The decay of the acoustic frequencies are monitored. A determination is then made as to the frequency that produced a standing wave. From that frequency, the level of the material n the tube is determined. Beneficially, acoustic frequencies are not produced in the body of the tube.